"this" in javascript

In Javascript, the key work "this" is a hard point to understand, in this artical, I will try to explain it

talking about "this", we have to talk about closure and the way function is called in Javascript.

function have for kinds of calling:

1. called as a method:

    var s = {
        a : '123',
        foo:function(){
            return this.a
        }
    }
    
    s.foo() //123

in this code, the function is called as object's method, in the closure of the function ,this point to its parent object.

2. called as a function

var a = 123;
var foo = function() {
	var a = 456;
	return this.a
}
foo() // 123

In this code, the function is called as a function, in the code's closure, this will point to the gloable window.

3. called as a constructor

    var Foo = function(){
        this.a = 1;
        this.b = 2;
        return this;
    }
    
    var foo = new Foo();
    foo.a //1;

 in this code, the function is called as a constructor, this will point to the new object.

4. called by .apply and .call

    var foo = function(arg1){
        this.a = arg1;
    }
    var s = {};
    foo.apply(s,[1]);

 in this code, when the apply is called, this will be binded to the first param of the apply.